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Date: Wed, 12 Jul 2017 15:41:05 +0200
From: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
To: Bandan Das <bsd@redhat.com>
Cc: Jim Mattson <jmattson@google.com>,
        David Hildenbrand <david@redhat.com>, kvm list <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 3/3] KVM: nVMX: Emulate EPTP switching for the L1
 hypervisor
Message-ID: <20170712134105.GG3326@potion>
References: <20170710204936.4001-1-bsd@redhat.com>
 <20170710204936.4001-4-bsd@redhat.com>
 <2d50ebc4-9328-ce08-b55b-6a331ee13cc3@redhat.com>
 <jpgy3ruam9i.fsf@linux.bootlegged.copy>
 <CALMp9eQyWQyyyP=K1+HieiQJHvBFzMEFGufOz6BFgb-mmdQm=Q@mail.gmail.com>
 <jpga84ayg8b.fsf@linux.bootlegged.copy>
 <20170711191334.GB3442@potion>
 <jpg8tjuydax.fsf@linux.bootlegged.copy>
 <20170711202227.GC3442@potion>
 <jpgwp7e8zyw.fsf@linux.bootlegged.copy>
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2017-07-11 16:45-0400, Bandan Das:
> Radim Krčmář <rkrcmar@redhat.com> writes:
> 
> > 2017-07-11 15:38-0400, Bandan Das:
> >> Radim Krčmář <rkrcmar@redhat.com> writes:
> >> 
> >> > 2017-07-11 14:35-0400, Bandan Das:
> >> >> Jim Mattson <jmattson@google.com> writes:
> >> >> ...
> >> >> >>> I can find the definition for an vmexit in case of index >=
> >> >> >>> VMFUNC_EPTP_ENTRIES, but not for !vmcs12->eptp_list_address in the SDM.
> >> >> >>>
> >> >> >>> Can you give me a hint?
> >> >> >>
> >> >> >> I don't think there is. Since, we are basically emulating eptp switching
> >> >> >> for L2, this is a good check to have.
> >> >> >
> >> >> > There is nothing wrong with a hypervisor using physical page 0 for
> >> >> > whatever purpose it likes, including an EPTP list.
> >> >> 
> >> >> Right, but of all the things, a l1 hypervisor wanting page 0 for a eptp list
> >> >> address most likely means it forgot to initialize it. Whatever damage it does will
> >> >> still end up with vmfunc vmexit anyway.
> >> >
> >> > Most likely, but not certainly.  I also don't see a to diverge from the
> >> > spec here.
> >> 
> >> Actually, this is a specific case where I would like to diverge from the spec.
> >> But then again, it's L1 shooting itself in the foot and this would be a rarely
> >> used code path, so, I am fine removing it.
> >
> > Thanks, we're not here to judge the guest, but to provide a bare-metal
> > experience. :)
> 
> There are certain cases where do. For example, when L2 instruction emulation
> fails we decide to kill L2 instead of injecting the error to L1 and let it handle
> that. Anyway, that's a different topic, I was just trying to point out there
> are cases kvm does a somewhat policy decision...

Emulation failure is a KVM bug and we are too lazy to implement the
bare-metal behavior correctly, but avoiding the EPTP list bug is
actually easier than introducing it.  You can make KVM simpler and
improve bare-metal emulation at the same time.